Sevens

Easy to learn card game! Get rid of all cards before your opponents do.
Players take turns playing cards - only cards adjacent in rank to those already played can be played. Sevens can always be played. If a player has no playable card, s/he must take one from the player to the left.
When you've tired on the AI it's a lot of fun to play this with real cards against real friends. :-)
This version does not include hard AI; a paid version will be released late in 2011 if the free version generates interest.